A thermohydraulic modelling for the stator bars of largeturbogenerators: development, validation by laboratory and on site tests

This paper presents a thermohydraulic model of stator bars in large turbogenerators. From the losses dissipated in a bar and considering the heat transfer by conduction and convection, it calculates the hot spot temperatures in it and the slot thermocouple temperature, for a clean or polluted stator. This model has been validated by laboratory tests made on individual bars and also by on site measurements made on a 900 MW turbogenerator. It shows the difficulty for the detection of hot spots by a slot thermocouple, and its systematic detection by the water temperature measurement at the bar outlet end. The modelling may be applicable for water-cooled machines ranging from 600 MW up to 1500 MW     

Stability of ZnMgO oxide in a weak alkaline solution

Zinc oxide (ZnO) is a chemical compound of great interest used, for example, as photocatalyst in the purification of wastewater or polluted air. However, neither dissolution, nor photo-dissolution of ZnO is negligible: indeed, both processes reduce significantly the efficiency of photocatalysis and then lead to a secondary pollution by free Zn^2 +. In the present study, the stability of ZnMgO thin films in weak alkaline solution is investigated. We demonstrate that the replacement of Zn^2 + ion with Mg^2 + ion results in the production of a Zn_0.84Mg_0.16O solid solution, whose stability is higher than that of the ZnO sample. This alloy, thus, constitutes an alternative to the use of ZnO in photocatalysis applications. To gain more insights into the higher resistance of such alloys to the dissolution process, X-Ray photoelectron spectroscopy measurements were performed. They highlighted the role of OH group adsorption in the experimentally observed enhancement of ZnMgO stability.     

Regulation of a PWM rectifier in the unbalanced network state usinga generalized model

This study concerns the modeling and control of a pulse-width-modulated (PWM) rectifier in the case of network variations. The aim is to limit and stabilize variations of DC output voltage and line currents in such circumstances. Network variations can result in costly damage to power converters and their loads but a power converter such as the PWM rectifier, using cascade digital control, offers many capabilities to stabilize the system with optimized control. A generalized model of the PWM rectifier is first presented using the Clarke notation in order to separate the positive and negative sequences. The model is also extended to the harmonics. The cases of harmonic disturbance and an unbalanced network are then analyzed and an optimized regulation is presented for the latter case, validating the generalized model. Experimental results are proposed. The line current compensation loop method coupled with identification of network parameters offers a good solution to stabilize the PWM rectifier in an unbalanced network     

Measurements and computer simulation of fast transients throughindoor and outdoor substations

The high-frequency modeling of substations is discussed. The response of three different substations (both indoor compartmented substations and outdoor substations) to fast front overvoltages (lightning overvoltages) has been measured by a low-voltage injection technique. Detailed modeling of substations, including busbars, circuit breakers, and disconnectors, has been derived from the test results. EMTP (electromagnetic transient program) simulations were run using these models, and in most cases very good agreement has been obtained with another independent series of tests     

A link between EMTP-RV and FLUX3D for transformer energization studies

This paper presents a programmed link between the electromagnetic transients program EMTP-RV and the finite element field solver FLUX3D. The model created in FLUX3D is driven from simulation designs in EMTP-RV. The test cases presented in this paper demonstrate that the coupling method is numerically robust and with sufficient accuracy. This approach benefits from EMTP advantages in modeling large-scale networks and from field solver advantages for detailed representation of power transformer iron cores.     

Characterization of the fragmentation of protein aggregates in suspension subjected to flow

Suspensions of flocculated aggregates subjected to flow present two distinct typical fragmentation mechanisms: cleavage and erosion. In this paper, a mathematical model is developed to concurrently evaluate the two phenomena. Erosion is tackled by a perturbative calculation, while cleavage is treated according to an exact calculation. The model is implemented on experimental data on the fragment size distribution in suspension of protein aggregates subjected to flow. Both cleavage and erosion are seen to gradually rise in significance as the flow rate increases. Their respective extent are each quantified with one chosen single parameter: cleavage with the number of fragments and erosion with the amount of eroded material. A relation found between the two parameters indicates a proportionality between the amount of eroded material and the surface area newly exposed by the process of cleavage.     

Ray casting with "on-the-fly" region growing: 3-D navigation into cardiac MSCT volume.

This paper describes an extended ray casting scheme for three-dimensional (3-D) navigation into the heart cavities for preoperative planning using multislice X-ray computed tomography data. The key benefit is that artifacts due to contrast inhomogeneities can be eliminated during volume traversal, thus improving the visual perception of the endocardial wall.     

Presentation of a system for the improvement of the on-line thermalmonitoring on 900 MW turbogenerators for predictive maintenance purposes

This paper firstly presents an acquisition system which has been developed in order to rapidly detect the overheating in the stator bars of large turbogenerators in nuclear power plants; it has been installed in 1994 on the stator of Tricastin. Secondly, associated to it, the development of a new monitoring system is presented, based on the determination of the fingerprints of the stator from the thermal modelling of the water cooling circuit. Validated by the measurements made on site, it also enables the following in real-time of the status of the machine, provides a detailed diagnosis of it, and optimises the load in abnormal conditions. It may turn the actual maintenance made on those machines towards the concept of predictive maintenance